This invention relates to electrodeposition (hereafter referred to as ED) coating systems and methods, and more particularly to ED coating systems/methods utilizing a first electrode which is to be coated, and plurality of second electrodes provided in association with the first electrodes.
ED coating generally may be broadly divided into two categories, including one using a coating material of an anion type and the other using a coating material of a cation type. Since, in either of these ED coatings, uniformity and adhesion of the coating on an article to be coated are excellent and the degree of pollution is generally low, these ED coating techniques have been widely applied recently to prime coating or one coat finishing of metal materials, such as automobile vehicle bodies.
As for the coating materials used in such ED coatings, as a coating material of an anion type, for example, a resin of molecular weight of 2000 often is used to which a carboxyl group is attached to make it water soluble; in the case of a coating material of a cation type, an amino group is attached to the resin to make it water soluble. Even with these water-soluble coating materials, however, the degree of ionization after being dissolved in water is very low. For this reason, at present, in the case of the coating material of an anion type, an alkaline neutralizing agent such as tai-ethylamine, for example, is mixed thereinto, while, in the case of the coating material of a cation type, an acidic neutralizing agent such as acetic acid is mixed thereinto. In both cases, neutralizing is effected, respectively, to thereby increase the degrees of ionization in the water.
As seen above, neutralizing agents are added and mixed to increase the degree of ionization in accordance with the properties of the resin components of the respective coating material. On the other hand, when the ED coating on the articles to be coated proceeds thereby decreasing the resin component in the solution, the coating material should be successively supplied from outside. Accordingly amine or acetic acid, as the neutralizing agent, accumulate in the solution, whereby a phenomenon such as redissolving of the coated film or pinholes occurs, so that the efficacy of the ED coating is impaired to a considerable extent.
For this reason, as described in Japanese Patent Kokoku (Post-Exam. Publn.) No. 22231/1970, for example, so-called pH control is performed to increase the efficiency. By such a method that a second electrode is separated from the article to be coated and from the aqueous solution by use of an ion-exchange membrane or the like, amine or acetic acid are osmotically extracted, to thereby prevent the accumulation of neutralizing agent in the aqueous solution.
ED coating of a cation type using a coating material of a cation type will be hereunder described. In ED coating of a cation type an anion exchange membrane has been used as a membrane. This anion exchange membrane normally has an efficiency of 8-10xc3x9710xe2x88x926 (mole/Coulomb) as an electric efficiency of removing the acid (coulombic acid removing rate). The acid (neutralizing agent) added to the aqueous solution (ED bath coating material) in the electrodeposition coating bath amounts to a value A contained in the coating material that is supplied to the ED bath.
On the other hand, the total amount of the acid taken out from the ED bath coating material to the outside equals a value B, which includes: (1) 10-20% of the value A taken out as acid contained in a UF filtrate which is used as a rinsing liquid after the ED coating; (2) 5-10% of the value A taken out as acid contained in the coated film; and (3) 70-80% of the value A, which is removed by the membrane electrodes. Although it is ideal that the value A is equal to the value B, it is difficult to adjust in order to obtain such an equality by conventional techniques. In general, B greater than A is adopted, whereby, if needed, a small amount of acid is added to the bath to keep a generally more exact acid balance.
For such reasons, when all of the electrodes provided in the electrodeposition bath happen to be the membrane electrodes for extracting acid, removal of the acid becomes highly excessive, whereby such disadvantages are presented that the acid as being the neutralizing agent lacks and the acid needs to be periodically supplied from the outside and so forth, so that the control of the neutralizing agent in the ED bath coating material becomes troublesome and the acid is uselessly consumed. For this reason, sometimes some of the electrodes are replaced with so-called bare electrodes having no membranes, or with membrane electrodes having extremely low acid removal rate, so that removal of the acid can be better balanced.
As described above, when the rate of removal is 8-10xc3x9710xe2x88x926 (mole/Coulomb), removal of the acid becomes excessive and when the rate of removal is 5-6xc3x9710xe2x88x926 (mole/Coulomb), removal of the acid becomes more nearly ideally balanced, so that a neutral membrane having the latter rate of acid removal may be used sometimes.
The above-mentioned conventional techniques require, in the event that the acid concentration in the ED bath becomes too low, to add acid directly from outside. There is, however, a disadvantage in this method as such work of addition of acid not only requires labor but also it is quite dangerous. Further, there is an additional problem with such techniques in that there is a sudden change in the acid concentration between before and after addition of acid, which tends to cause abrupt change in the paint characteristic.
The present invention aims to provide ED coating systems and methods which eliminate such problems of conventional techniques and provide a new technique, with interest paid to the function of acid removal of membrane electrodes, that enable adjustment without directly adding acid from outside when acid concentration in the bath tends to go too low.
To attain the goal mentioned above, an ED coating method is proposed which comprises a first electrode as an article to be coated provided in an ED bath and a plurality of second electrodes provided in association with the first electrode, wherein current is passed between the article to be coated and the second electrodes through an aqueous solution of a substance contained in the electrodeposition bath, to thereby electrodeposit the substance for forming a coating film onto the article to be coated, and the second electrodes comprise a number of membrane electrodes having a membrane portion which separates the electrode from the aqueous solution. Some of these second electrodes are a low acid removal type electrode, each of which is provided with corrosion resistant electrode material and first membrane portion having a function of precluding most of the flow of ionized neutralizing agent in the aqueous solution from being extracted, and the remaining second electrodes are high acid removal type membrane electrodes being each provided with a second membrane portion having a function of osmotically extracting the neutralizing agent, wherein these low acid removal type membrane electrodes and high acid removal type of membrane electrodes are placed along the bath paint tank wall.
Further each of the high acid removal type membrane electrodes is provided with a first electrolyte circulation system to run electrolyte from one end to the other end between its second type membrane and electrode pipe, likewise each of the low acid removal type membrane electrodes is provided with a second electrolyte circulation system functioning basically the same as the first electrolyte circulation system, independently from the first system. Both of the first and second electrolyte circulation systems are provided with correspondingly first and second conductivity control circuits/units which are activated if the conductivity exceeds a pre-set reference conductivity value in order to controllably introduce D.I. water to corresponding electrolyte as dilution media, where the second conductivity control circuit/unit has a higher preset reference value of conductivity than that of the first conductivity control circuit/unit above which reference conductivity point D.I. water is introduced into the electrolyte.
Also in accordance with certain embodiments of the present invention, a DC voltage is applied in such a way that the article to be coated is connected to a negative pole and each of the membrane electrodes (second electrodes) is connected to a positive pole. Immediately, ED coating starts, and the positively charged paint resin and pigment colloids in the aqueous solution start to migrate toward the article to be coated which is negatively charged, forming a coating film on its surface, while leaving negatively charged acid (acetic acid) in the aqueous solution.
In this case, as mentioned above, as soon as ED coating starts the acid (acetic acid), as a neutralizer, starts to migrate toward the second membrane electrodes. The acid, however, will be mostly precluded by the membrane electrodes with cation ion-exchange membranes, and, as a result, if it is left alone acid will accumulate in the aqueous solution. On the other hand, as other of the second membrane electrodes have second membranes which pass acetic acid molecule easily, acid molecules which are attracted to these positive electrodes, will pass this anion exchange membrane along the line of electric field force. As a result, acid is gathered between the anode and membrane, which is carried out by the flowing out of the electrolyte. In this way, acid will not accumulate excessively in the aqueous solution. Generally, acid is carried out excessively from the paint bath and acid in the bath rather tends to be depleted.
D.I. water is circulated in the first and second electrolyte circulation systems as a closed loop, and acid concentration starts to rise as the ED process continues. This will result in lowering the electric resistance of electrolyte (conductivity will rise). In this situation, the mentioned conductivity control circuit/unit is activated, namely if the conductivity of the electrolyte in first and second electrolyte circulation systems surpass the set conductivity values, then the conductivity control device will supply electrolyte with D.I. water as a dilution media. As the conductivity of the electrolyte will go down by the addition of D.I. water (electric resistance of electrolyte increase), the electric current to the first membrane electrode (high acid removal type of electrodes) will decrease and removal of acid will go down. By this the excessive extraction of acid from the bath paint is avoided, thereby helping to keep the acid concentration in proper level. At the same time, corrosion of the anode by acid is suppressed in each of these membrane electrodes which are connected to the first electrolyte circulation system.
By setting the conductivity of the second electrolyte circulation system at a high value, the conductivity of the electrolyte of the second electrolyte circulation system is kept on average higher (resistance is on average lower) than that of the first. Thus, the electric current flow to the electrodes connected to the second electrolyte circulation system (low acid removal type of membrane electrodes) becomes higher than the electric current flowing to the electrodes connected to the first electrolyte circulation system (high acid removal type of membrane electrodes). As a result, the membrane electrodes connected to the second electrolyte circulation system (low acid removal type membrane electrodes) are controlling the membrane electrodes connected to the first electrolyte circulation system (high acid removal type membrane electrodes), and by so doing it is effectively suppressing the extraction of excessive acid from the bath paint in the ED coating tank.
In accordance with the present invention, we also can propose such an arrangement that the first and second electrolyte control circuits/units each are provided with, correspondingly, first and second conductivity probes which monitor conductivity of the electrolyte of first and second electrolyte circulation systems, respectively, and first and second DI water supply devices to controllably add a desired or set amount of DI water, as dilution media, to the first and second electrolyte circulation systems, and a first and second D.I. water supply control part to send a signal to the first and second water supply devices when conductivity exceeds the pre-set conductivity reference value to activate the DI water supply devices, wherein the first and second D.I. water supply control parts have correspondingly first and second parts to set or change the conductivity reference value of activation.
In accordance with the present invention, it becomes possible, and gives advantage, not only to secure the stability of function of each electrolyte conductivity control, but also, in case acid is extracted excessively from the bath paint, to respond quickly to control the acid in the bath paint for a long time. Depending on the demands of the situation, the activation reference value of the second D.I. water control part can be changed with the second reference value setting part, thus changing the timing of D.I. water supply to the low acid removal type membrane electrodes which in turn changes the electric current that flows to the high acid removal type membrane electrodes, which in turn indirectly control the acid removal of high acid removal type membrane electrodes.
We can further propose such an arrangement that the first and second electrolyte circulation systems each correspondingly has first and second electrolyte tanks to hold a predetermined or set amount of electrolyte, piping between the first electrolyte tank and low acid removal type membrane electrodes and piping between the second electrolyte tank and high acid removal type membrane electrodes, and correspondingly first and second pumps and first and second valves built into this piping, wherein the first and second electrolyte circulation systems have correspondingly first and second control parts to control correspondingly the first and second pumps and valves, while each of the low acid removal type membrane electrodes and high acid removal type membrane electrodes preferably are grouped together through headers for electrolyte supply and return. In such a manner, the first and second electrolyte tanks and headers work as a flow buffer, and more efficiently maintain smooth circulation when there is some pressure difference in the different part of piping, or when air bubbles are trapped in the electrolyte flow.
Also in accordance with certain embodiments of the present invention, an arrangement may be provided in which a first electrode as an article to be coated is provided in an ED bath and a plurality of second electrodes are provided in association with the first electrode, wherein current is passed between the article to be coated and the second electrodes through an aqueous solution of a substance contained in the ED bath, to thereby electrodeposit the substance for forming a coating film onto the article to be coated, wherein each of the second electrodes comprises an electrode and membrane which separates the second electrode from the aqueous solution. In accordance with the present invention, some (e.g., a first group) of the second electrodes are low acid removal type electrodes, each being provided with corrosion resistant electrode material and first type membrane having a function of precluding most of the flow of ionized neutralizing agent in the aqueous solution from being extracted, and the rest (e.g., a second group) of the second electrodes being high acid removal type electrodes each being provided with a second membrane having a function of osmotically extracting the neutralizing agent. Each of the high acid removal type membrane electrodes preferably is provided with a first electrolyte circulation system to run electrolyte from one end to the other end between its second type membrane and electrode pipe, likewise each of the low acid removal type membrane electrodes is provided with a second electrolyte circulation system functioning basically the same as the first electrolyte circulation system, independently from the first system.
Further in accordance with embodiments of the present invention, the first electrolyte circulation system is provided with a first electrolyte conductivity control circuit/unit (e.g., control means), which functions to control the conductivity of circulating electrolyte solution by adding a quantity of D.I. water for dilution so to keep its electrolyte conductivity within a predetermined or set range, and the second electrolyte circulation system is provided with a second electrolyte conductivity control circuit/unit which functions to control conductivity of the second electrolyte below a set value by adding D.I. water when its conductivity exceeds a predetermined or pre-set reference value, and continue until the conductivity gets down below the predetermined or preset reference conductivity value. Further, in preferred embodiments of the present invention the pre-set activation reference value of the second electrolyte conductivity control circuit/unit is set greater than the maximum value of the conductivity range of first electrolyte conductivity control circuit/unit.
With such embodiments as disclosed herein, there is also an advantage of securing stable work of the control system. By providing the first electrolyte circulation system with the capability of setting a range of conductivity of electrolyte it can respond with a certain range of conductivity (range of tolerance) and avoid chattering which may occur when there was rapid change of up and down conductivity. As a result, such embodiments provide a capability to control acid concentration in the ED bath paint. In this case it is possible to provide, as with the case of the first electrolyte conductivity control circuit/unit, a second electrolyte conductivity control circuit/unit with capability to keep the conductivity of the electrolyte of the second electrolyte circulation system within a pre-set range of conductivity. In preferred embodiments, it is advisable, in this case, to set the maximum and minimum of the conductivity range set with the second electrolyte conductivity control circuit/unit greater than those of the first electrolyte conductivity control circuit/unit. In such embodiments, such a method provides the advantage of avoiding chattering of the second electrolyte conductivity control circuit/unit when the conductivity of the second electrolyte circulation system fluctuate up and down, resulting in improved overall stability of the system.
Further, in accordance with the present invention such arrangement that the first and second electrolyte control circuit/unit each has correspondingly first and second conductivity probes which monitors the conductivity of the electrolyte of the first and second electrolyte circulation systems, and first and second DI water supply devices to add a predetermined set amount of DI water, as dilution media, to the first and second electrolyte circulation systems, and first and second D.I. water supply control parts which work by a signal from the first and second conductivity probes and thereby control first and second water supply devices, and these first and second D.I. control parts each have the capability to adjust the maximum and minimum value of conductivity range or a reference value. In accordance with the present invention, such a method secures and improves an independent and trouble free supply of D.I. water to the electrolyte of the above mentioned first and second electrolyte circulation systems, resulting in smooth automatic conductivity control of electrolyte.
Also in accordance with the present invention, an arrangement may be provided in which a first electrode as an article to be coated is provided in an electrodeposition bath and a plurality of second electrodes are provided in association with the first electrode, wherein current is passed between the article to be coated and the second electrodes through an aqueous solution of a substance contained in the electrodeposition bath, to thereby electrodeposit the substance for forming a coating film onto the article to be coated, wherein the second electrodes comprise an electrode and a membrane that separates the electrode from the aqueous solution.
In preferred embodiments, some of the second electrodes are low acid removal type electrodes, each of which is preferably constituted with a corrosion resistant electrode material and membrane having a function of precluding most of the flow of ionized neutralizing agent in the aqueous solution from being extracted, and the rest of the second electrodes are high acid removal type electrodes each of which is provided with a second membrane portion having a function of osmotically extracting the neutralizing agent, wherein a number of low acid removal type membrane electrodes and high acid removal type of membrane electrodes are placed along the bath paint tank wall, and each of the high acid removal type membrane electrodes is provided with a first electrolyte circulation system to run electrolyte from one end to the other end between its second type membrane and electrode pipe, likewise each of the low acid removal type membrane electrodes is provided with a second electrolyte circulation system functioning the same as the first electrolyte circulation system, independently from the first system.
Then, a probe is provided in the ED bath tank to measure the acid concentration in the bath paint, and the first and second electrolyte circulation systems are provided with correspondingly, and independently from each other, first and second conductivity control circuits/units which are activated if conductivity in the ED bath paint becomes lower than a predetermined or set reference point to controllably introduce a desired or set amount of D.I. water to either the first or second electrolyte circulation system as a dilution media.
In accordance with such embodiments, further advantages are provided, such as quicker and more direct response to a drop of the acid concentration in the ED bath paint, as it directly monitors the acid concentration in the ED bath paint. In accordance with the present invention, we can propose such an arrangement that the first and second electrolyte control circuits/units each has correspondingly first and second conductivity probes, first and second D.I. water supply devices, which supply a controlled or set amount of D.I. water, as dilution media, to the first and second electrolyte and first and second D.I. water supply control parts which control first or second D.I. supply devices depending on the information from the acid concentration probe in the ED bath paint or from first or second conductivity probes, wherein each of the first or second D.I. water supply control parts is provided with first or second parts to set or change the desired reference value. With such embodiments, it becomes possible to automatically control the acid concentration in the bath paint quickly and with stability. At the same time it can control the conductivity of the first and second electrolyte circulation systems, so that degradation of anodes in the membrane electrodes connected to these electrolyte circulation systems will be avoided.
In certain embodiments, a modification of an ED coating system is provided where the membrane electrodes are installed along the ED coating tank wall in such a way that high neutralizer removal type membrane electrodes are placed in the upstream (first) zone where the article to be coated is brought in and generally a first, low voltage is impressed, high neutralizer removal type membrane electrodes and low neutralizer removal type membrane electrodes are placed mixed in downstream (second) zone where generally a second, higher voltage is impressed. For this reason, with such embodiments, because both high acid removal type membrane electrodes and low acid removal type membrane electrodes are placed mixed together, the change of conductivity of the electrolyte in the two type of membrane electrodes will influence mutually and directly. Namely, if D.I. water was added to the electrolyte of one of the two types of membrane electrodes, and thus the conductivity is reduced (resistance is increased), then the conductivity of the electrolyte of the other type of membrane electrodes is relatively increased (resistance is decreased).
For this reason, a relatively greater part of the electric current of the ED coating flows to electrodes with lower resistance than electrodes with higher resistance. In this particular case under discussion, a relatively greater part of the electric current flows to second membrane electrodes (low acid removal type membrane-electrodes) than first membrane electrodes (high acid removal type membrane-electrodes). As a result, acid removal from the bath paint is controlled effectively without changing the total electric current flow, and leads to smoother management of acid concentration in the bath paint.
Here, it is also possible, in the high voltage zone, to have placement of a number of two kinds of membrane-electrodes, from upstream where the generally lower voltage is impressed to downstream where the generally higher voltage is impressed, in such a way as, for example, a zone with low acid removal type membrane electrodes only, a zone in which both types are mixed, and finally a zone with high acid removal type membrane electrodes. In this way acid control is mainly done in the center of the ED tank, but the paint is constantly mixed and, for the paint in bath as a whole, acid removal is balanced. Particularly for the zone where the two types of membrane-electrodes are mixed, it is preferred to place the two kinds alternatively one by one, or two by two. In this way the electric current can be divided between low acid removal type membrane electrodes and high acid removal type membrane electrodes in a more ideal ratio, while keeping total current to a desired level in regard to the size of article to be coated, as the two kinds of membrane electrodes are placed close to each other and alternatively. As a result, it is possible to control the amount of acid removed from the bath paint by adding D.I. water to either of the two electrolytes.
In alternative embodiments, as basic system construction, an ED coating method is provided that includes a first electrode as an article to be coated provided in an ED bath and a plurality of second electrodes provided in association with the first electrode, wherein current is passed between the article to be coated and the second electrodes through an aqueous solution of a substance contained in the electrodeposition bath, to thereby electrodeposit the substance for forming a coating film onto the article to be coated, wherein the second electrodes include at least two types of electrodes, namely bare electrodes preferably made of a corrosion resistant material, and membrane electrodes made of an electrode and a membrane which separates the electrode from the aqueous solution. Some of said membrane electrodes are high acid removal type membrane electrodes, which comprises membrane that osmotically extract neutralizer ion in bath paint, wherein a number (i.e., plurality) of the bare electrodes and high acid removal type of membrane electrodes are placed along the ED paint tank wall. Preferably, each of the high acid removal type membrane electrodes are provided with a first electrolyte circulation system to run electrolyte from one end to the other end between its second type membrane and electrode pipe, wherein the first electrolyte circulation system is provided with conductivity control circuit/unit (e.g., means) that keeps the conductivity of electrolyte within a predetermined or set range.
Additional advantages of such embodiments of the present invention include the advantage of low initial investment cost and simpler maintenance, as such embodiments may utilize corrosion resistant bare electrodes, in place of a number of second membrane-electrodes. Here, we propose that the bare electrodes and high acid removal type membrane electrodes are installed along the ED coating bath tank wall in such a way; in preferred embodiments, high acid removal type membrane electrodes are placed in the upstream (first) zone where a generally low (lower) voltage is impressed, and give an area, in a downstream (second) zone, where generally a higher voltage is impressed, where high acid removal type membrane electrodes and bare electrodes are placed in a mixed manner.
In still other embodiments, it is possible to install the bare electrodes and high acid removal type membrane electrodes alternately in the downstream area where the generally higher voltage is impressed. Further, it is also possible to have the bare electrodes and high acid removal type membrane electrodes installed alternately two by two (or n by n) in the downstream area where the generally higher voltage is impressed.